Cellular transplantation into the early gestational fetus is a promising strategy to treat congenital hematopoietic disorders. Theoretically, transplantation of foreign cells into the fetus, prior to the maturation of the immune system, could result in life-long tolerance to the donor. However, most clinical applications have not been successful, indicating there are multiple barriers to the engraftment of transplanted cells. The long-term objective of this research is to understand and overcome these barriers. In the mouse model of in utero hematopoietic stem cell transplantation (IUHSCTx), we and others have shown that the immune response of the host limits engraftment. However, we have recently demonstrated that transplants into normal fetuses carried by immunodeficient mothers are all successful, indicating that the maternal immune system is an important (and previously unrecognized) barrier to IUHSCTx. We have also shown that there is considerable trafficking of immune cells from the mother into the fetus during normal development. We hypothesize that the maternal immune system plays an essential role in limiting the success of IUHSCTx. Therefore, modulating this immune response may help to overcome a significant barrier to engraftment. In Specific Aim 1, we will determine whether maternal B or T cells are necessary for graft rejection. In Aim 2, we will study maternal/fetal cellular trafficking after fetal intervention to determine how maternal cells encounter donor cells to promote rejection. In Aim 3, we will test whether maternal tolerance induction improves engraftment. Understanding the role of the maternal immune system in fetal transplantation has important clinical applications for designing transplantation strategies to treat fetuses with congenital hematopoietic disorders such as sickle cell disease, thalassemias, or immunodeficiencies. Beyond these diseases, tolerance induction to a particular donor can be used for fetuses with anatomic anomalies requiring postnatal organ transplantation. Insights gained into the process of maternal/fetal cellular trafficking may also improve our understanding of maternal/fetal tolerance during normal and abnormal pregnancy. As a training grant, these studies will allow me to develop a strong foundation in immunology so that I can ultimately become an independent investigator studying stem cell transplantation in the fetal environment. My training setting, with experts in immunology and stem cell biology and strong institutional support, is ideal for achieving this goal. Relevance: We believe that transplanting stem cells into the fetus-before the immune system matures-can potentially allow us to treat congenital diseases and avoid toxic immunosuppression. While this strategy has had limited success in humans, we continue to explore various ways to improve our techniques. The aim of this grant is to understand the role of the mother's immune system in rejecting the cells transplanted into the fetus so that we can devise approaches to optimize acceptance.